Knife sharpener



FIG. 1 is a front, right perspective view of the knife sharpener showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a left side view thereof; and,

FIG. 6 is a top plan view thereof.

The bottom of the base of the knife sharpener is substantially flat and unornamented.

The particular shape of the periphery of the base of the knife sharpener, as well as other features shown in broken lines in FIGS. 1-7, do not form part of the claimed design. 

The ornamental design for a knife sharpener, as shown and described. 